1. Field of the Invention:
The present invention is directed to an improved method for the manufacture of optical fiber.
2. Description of the Prior Art:
During the past decade, a great deal of work has been undertaken directed to the manufacture of glass transmission lines for visible and near-visible electromagnetic radiation, commonly referred to as optical waveguides or optical fiber. In brief, optical waveguides are extremely thin, flexible glass fibers, prepared by drawing glass preforms under controlled conditions. Although these conditions are subject to wide variation, depending upon such factors as the composition of the glass and the drawing procedure, the optical waveguide so prepared will comprise, in its simplest form a glass core of a given constant diameter, surrounded by a glass cladding having an index of refraction less than that of the glass core. The cladding acts as a barrier which confines the light to the core.
While the drawing procedure is important in obtaining a properly functioning optical waveguide, it is the production of the glass preform which is drawn into the fiber which controls the ultimate composition and hence transmission properties of the waveguide. Thus far, workers in the art have proposed to manufacture such preform in several ways. One method, described in U.S. Pat. No. 3,932,160, forms a glass preform by deposition of borosilicate particulate on a pure silica start rod by high temperature pyrolysis of a gaseous mixture of silane and borane or a gaseous mixture of silicon tetrachloride and boron trichloride. The temperature required to effect the pyrolysis reaction can be produced by conducting the deposition in a reaction furnace which heats both the gaseous mixture of reactants and the rod. Alternately, the requisite temperatures can be obtained by directing the reactants through a natural gas flame in the presence of oxygen. The pyrolysis reaction produces fine particles of borosilicate glass which are deposited on the pure fused silica start rod. After the deposition is completed, the composite is introduced into a furnace where it is heated to sinter the deposited particulate, thereby forming the outer region of the core and the cladding of the preform.
To produce a preform having a graded profile,that is, a radially varying index of refraction, the ratio of the silicon containing reactant to the boron containing reactant in the gaseous mixture is varied in such a manner that borosilicate particulate having increasing proportions of boron oxide relative to silica is produced. Moreover, to insure that the deposition of borosilicate is uniform for a given radial distance from the preform axis, the start rod must be simultaneously translated and rotated to provide even distribution of the particulate over the length of the rod. Thus, by setting a uniform translation and rotation rate for the start rod, the concentration of components in the gaseous reactant mixture can be continously varied so as to achieve the desired radial concentration profile of borosilicate which is uniform for a given radial distance from the center of the rod.
Because the nature of optical fiber is such that slight discrepancies in the radial compositional profile may cause significant deleterious results in transmission capability, it is always desirable to employ a method for producing waveguides which insures that a precisely uniform composition profile is obtained.